1. Field of the Invention
The present invention relates to an electron gun and, more particularly, to an electron gun for a cathode ray tube (CRT) including a correction electrode having asymmetric beam through holes located between a grid having a single aperture and a shield cup.
2. Description of the Related Art
An electron gun for a color CRT generally includes a triode having cathodes, a first grid G1 and a second grid G2, a third grid G3 opposing the second grid G2 and forming a pre-focusing lens, a fourth grid G4 opposing the third grid G3 and forming a main lens, and a shield cup.
When power is applied to a cathode ray tube, the electron gun emits electron beams from the cathodes. The emitted electron beams are focused and accelerated while passing through apertures in a plurality of grids. The accelerated electron beams are selectively deflected by a deflection yoke installed on a cone portion of a bulb of the CRT and excite phosphors on a screen, thereby producing a displayed image. Electron guns have various structures for correcting errors in convergence of electron beams landing on peripheral parts of the screen due to the non-uniform deflecting magnetic field of the deflection yoke.
FIG. 1 is a horizontal sectional view showing an electron gun 10 disclosed in U.S. Pat. No. 5,517,078, FIG. 2A shows a third grid G3 shown in FIG. 1, and FIG. 2B shows a fourth grid G4 shown in FIG. 1. As shown in FIGS. 1, 2A, and 2B, the electron gun 10 includes three cathodes, KR, KG, and KB, first through fourth grids, G1 through G4, sequentially arranged in the direction of a phosphor screen, and a convergence cup Cp on the fourth grid G4. In the third grid G3 shown in FIG. 2A, three beam through holes 31R, 31G, and 31B are arranged along a straight line on a surface opposing the fourth grid G4.
Among the beam through holes 31R, 31G, and 31B of the third grid G3, the center beam through hole 31G has a circular shape. However, each of the side beam through holes 31R and 31B has an elongated shape, elongated in a horizontal direction, that is, the X-axis direction, of the third grid G3. Opposite edges of each of the side beam through holes 31R and 31B are arcs A1 and A2, respectively having radii R1 and R2. The arcs A1 and A2 are connected to each other with straight edges L1 and L2. The length of the inner arc A1 toward the center beam through hole is greater than that of the outer arc A2.
The fourth grid G4, shown in FIG. 2B, includes three beam through holes 41R, 41G, and 41B, arranged along a straight line on a surface opposing the third grid G3. The beam through holes 41R, 41G, and 41B of the fourth grid, G4, are all circular. Among these beam through holes 41R, 41G, and 41B, side beam through holes 41R and 41B are slightly off-center, outwardly in the arrangement direction of the three electron beams, by a distance xcex94S with respect to the side beam through holes 31R and 31B of the third grid G3.
In the electron gun 10 having the described configuration, side beam through holes having inner and outer arcs of different lengths are located on at least one of the surfaces of the third grid G3 and the fourth grid G4 that face each other. Each of the third grid G3 and the fourth grid G4, forming a main lens, has three beam through holes. Thus, when forming asymmetric side beam through holes 31R and 31B, the effective individual aperture is reduced, thereby increasing spherical aberration. The main lens is very sensitive to alignment during assembly of the electron gun 10. The described grid configuration cannot ensure reliability of the electron gun 10. Also, minute adjustment of convergence is difficult.
FIG. 3 is a longitudinal sectional view of an electron gun 30 disclosed in U.S. Pat. No. 4,678,964. Referring to FIG. 3, the electron gun 30 includes three cathodes 31a, 31b, and 31c, a first grid 32, a planar second grid 33, a third grid 34, and a fourth grid 35. The third grid 34 includes cup-shaped parts 34a and 34b having open ends fixedly sealed to each other. The fourth grid 35 includes three beam through holes 35a, 35b, and 35c. Also, the fourth grid 35 further includes a cup-shaped field correction element 36 having rectangular beam through holes 36a, 36b, and 36c. The beam through holes 36a, 36b, and 36c of the field correction element 36 face the beam through holes 35a, 35b, and 35c. The field correction element 36 has a flange 36d connecting the fourth grid 35 and a sleeve 37.
The field correction element 36 is installed inside the fourth grid 35 and the beam through holes 36a, 36b, and 36c are vertically or horizontally elongated. Alternatively, the field correction element 36 is part of the grids 34 and 35, each of which has three beam through holes. Accordingly, the effective individual aperture is reduced, exhibiting a weak astigmatism correction. Because of the weakness of the correction, the improvement in distortion of beam spots at the peripheral portion of the screen is insufficient.
FIG. 4A is a front view of an electrode 40 disclosed in Japanese Unexamined Patent Application 2000-67774, FIG. 4B is a plan view of FIG. 4A, and FIG. 4C is a side view of FIG. 4A. In FIGS. 4A, 4B, and 4C, the electrode 40 is located between grids and a shield cup. The electrode 40 has three circular beam through holes 42, 43, and 44 arranged along a straight line on a planar portion 41. Perpendicular portions 45 and 46 are located at opposite edges of the planar portion 41. The electrode 40 has sloping portions 47.
The plate-shaped electrode is installed in the rear of a main lens for horizontal focusing and vertical divergence for improving performance of a quadrupole lens. However the electrode 40 is not reliable because it has perpendicular portions. Also, it is quite difficult to overcome the distortion of beam spots caused by side beam through holes 42 and 44.
To achieve the above object, there is provided an electron gun for a cathode ray tube including a triode having cathodes, a first grid, and a second grid; at least one third grid having a single aperture through which R, G, and B electron beams emitted from the cathodes commonly pass; a fourth grid opposing the third grid and forming a main focus lens with the third grid; a shield cup connected to the fourth grid and supplying a high voltage to the fourth grid; and a correction grid disposed between the fourth grid and the shield cup and having R, G, and B beam through holes with respective centers lying along a first line, the R and B beam through holes having respective openings that are asymmetrical about respective second lines, transverse to the first line, and passing through the centers of the R and B beam through holes, respectively.
Each of the R and B beam through holes of the correction grid may have an inner part near the center G beam through hole side that is longer than an outer part at the opposite side of the R and B beam through holes.
The R and B through holes may have edges describing trapezoidal openings.
The correction grid may have a planar surface facing the fourth grid and sloping surfaces facing the shield cup so that the correction grid has a thinnest part at the G beam through hole and becomes thicker, along the first line, toward each of opposite ends of the correction grid.
The correction grid may be a plate in which circular R, G, and B beam through holes are arranged along a straight line, and members for varying the openings of the R and B beam through holes are mounted on the plate blocking part of the R and B beam through holes, respectively.
According to another aspect of the invention, an electron gun for a cathode ray tube comprises a triode having cathodes, a first grid, and a second grid; at least one third grid through which R, G, and B electron beams emitted from the cathodes pass; a fourth grid opposing the third grid, forming a main focus lens; and a shield cup connected to the fourth grid and supplying a high voltage to the fourth grid and including R, G, and B beam through holes with respective centers lying along a first line, the R and B beam through holes having openings that are asymmetrical about respective second lines, transverse to the first line, and passing through the centers of the R and B beam through holes, respectively.
According to a third aspect of the invention, an electron gun for a cathode ray tube includes a triode having cathodes, a first grid, and a second grid; at least one third grid through which R, G, and B electron beams emitted from the cathodes pass; a fourth grid opposing the third grid and forming a main focus lens with the third grid; a shield cup connected to the fourth grid and supplying a high voltage to the fourth grid; and a correction grid disposed between the fourth grid and the shield cup and having R, G, and B beam through holes with respective centers lying along a first line, wherein the R and B beam through holes have respective trapezoidal openings that are symmetrical about the first line and asymmetrical about respective second lines, transverse to the first line, and passing through the centers of the R and B beam through holes, respectively.
According to a fourth aspect of the invention, an electron gun for a cathode ray tube includes a triode having cathodes, a first grid, and a second grid; at least one third grid through which R, G, and B electron beams emitted from the cathodes pass; a fourth grid opposing the third grid and forming a main focus lens with the third grid; a shield cup connected to the fourth grid and supplying a high voltage to the fourth grid; and a correction grid disposed between the fourth grid and the shield cup, and comprising a plate having circular R, G, and B beam through holes with respective centers lying along a first line, wherein the R and B beam through holes have respective openings in the correction grid that are asymmetrical about respective second lines, transverse to the first line, and passing through the centers of the R and B beam through holes, respectively, and the correction grid further includes members covering parts of the R and B beam through holes and mounted on the plate to produce the openings.